ISKeyValueList.ToCsvString; not handling "Nothing" strings

There is an issue in the Business Layer ISKeyValueList object.

Using the "SetString" method and passing in Value as an uninitialised string, i.e. Nothing, causes the "ToCsvString" function to repeat the value for the previous Key as the value.

The following changes have been made:

1. ISKeyValueList.SetString now correctly handles value = Nothing string parameter by changing it to a blank string.

2. The ToCsvString function now includes all standard key/ value pairs - regardless of whether the Value is blank.